138 proceedings: philosophical society 



was made the basis of that work. The causes which led to the estab- 

 lishment of the International Bureau of Weights and Measures in France, 

 and the prominent part taken by geodesists in the development and 

 preservation of standards of length up until very recent times, were 

 brought out. The progress made in the measurement of primary 

 base-lines by the U, S. Coast and Geodetic Survey during the period 

 while the speaker was connected with that service was then briefly 

 sketched, emphasis being laid upon the important part taken by mem- 

 bers of the Philosophical Society in the development of the subject, 

 not only in the United States, but throughout the world. The com- 

 pensated base bars of Schott, the simple steel bars used by Tittmann, 

 the duplex apparatus of Eimbeck, and the methods of Woodward of 

 measuring with the 5-meter iced bars, and steel tapes, were briefly 

 discussed and the advantages and the accuracy of the diff'erent methods 

 compared. Mr. Fischer also exhibited some very accurately measured 

 steel gauges to illustrate the progress in perfecting accurate manufac- 

 turers' standards. 



The chair expressed to the speaker the thanks of the members and 

 guests present for his most interesting paper. 



The 751st meeting was held on January 30, 1915, at the Cosmos 

 Club, President Eichelberger in the chair; 52 persons present. 



Mr. W. F. G. SwANN presented a paper on The atmospheric-electric 

 work of the Department of Terrestrial Magnetism, illustrated with lan- 

 tern slides. A brief survey of the methods and uncertainties in atmos- 

 pheric-electric observations was given, and certain new instruments 

 and methods were described. A short discussion of the observations 

 made by Messrs. C. W. Hewlett and H. F. Johnston during the sec- 

 ond and last cruises of the Carnegie was given. Ocean observations 

 appear to show a curious diminution in the conductivity followed by 

 an increase as one passes from mid-ocean to land, and vice versa. 

 Reasons were given for believing that the specific velocity is the factor 

 responsible for this phenomenon. The rg-dioactive content was de- 

 termined by a modification of Elster and Geitel's method. By an 

 application of theoretical considerations, it has been possible to calcu- 

 late the approximate absolute radioactive content of the air. The 

 decay curves correspond to radium emanation, and the average ema- 

 nation content per cu. cm. is 107 curies, which is as much as is found 

 on land, and would be more than sufficient to account for the con- 

 ductivity observed if many of the ions produced were not ions of slow 

 velocity. In the discussion Mr. Humphreys referred to the chaotic 

 state of atmospheric-electric work until recent years; the prog- 

 ress being made results first from a thorough discussion of the un- 

 derlying principles and then a design of apparatus accordingly. Mr. 

 Bauer referred to his experience in planning for the atmospheric- 

 electrical work of the Department of Terrestrial Magnetism in 1905, 

 and of the observational quarters being now provided on the Carnegie 

 for the coming cruise. 



